Manufacture of chemical wood pulp



Patented Dec. 27, 1927. y

UNITED STATES PATENT OFFICE.

MANUFACTURE OF CHEMICAL WOOD PULP.

No Drawing. Application filed January 29, 1925,

This invent-ion relates to improvements in the manufacture of chemical wood pulp, and in particular to the manufacture of so-called sulphite fiber by the cooking of wood chips and of similar vegetable fibers by bisulphite-solutions, usually of lime, magnesia and the like, and it is intended to devise means for facilitating the cooking process, and for improving the quality,

colour and strength of the fiber thereby obtained. As is well known, the usual procedure of manufacturing chemical pulp from wood or similar vegetable fibrous material is characterized by the fact that the raw material in the comminuted condition is boiled for a suflicient length of time by bisulphite solutions. In this cooking proc- "ess a certain maximum pressure according to the different requirements of the particularcase should not be exceeded. Whenever this admissible limit of pressure is exceeded in thecooking operation which is always effected by passing steam into the digesters, a reduction of pressure is produced by opening a valve provided in the upper portion of the digesterr Through this valve a certain proportion of the gas eous sulphurous acid and corresponding, though small quantities of the liquid contents of the digester are ejected and the pressure in the interior of the digester is lowered below the admissible maximum pressure, which maximum, however, is again exceeded after some time, andis again followed by an opening of the'valve and ejection of sulphurous acid with a corresponding reduction of pressure. This procedm'e is continued, until the pressure in the digester has ceased to go beyond the admissible highest limit. In this stage the digester is allowed to remain quiet for sometime, and then the contents vof the digester are boiled to the finish with the continued admission of steam. The increase of pressure in the digester. is eflected by the sulphurous acid, liberated by the increase of temperature and by the mechanical agitation of the liquor, and by the pressure of the saturated steam corresponding to the particular temperature, and by the quantity of air liberated from the pores of the wood during the cooking process. The heating of the entire contents of the digester to the end-temperature of, for instance, say 138 degrees C. necessitates the consumption of a considerable amount of steam, and itis of the digester.

Serial No. 5,629, as in Germany April 30, 1924.

well known that the commercial success of the manufacture of wood pulp depends cluefiy upon the quantities of steam required.

In accordance with my invention it has been ascertained that it is possible to reduce the amount of steam required to a fraction of the amount necessary according to the previous method, and yet'to produce a pulp of a better quality than in the ordinary "process by letting corresponding amounts of the liquor flow away through a valve at the bottom of the di ester for the reduction of pressure require instead of causin the liberated sulphurous acid to escape t1rough a valve provided at the top The amount of liquor which is removed in this manner, contains approximately the same quantity of sulphurous acid, which is allowed to escape in the previous method by the relieving operation ,through the upper valve. The removal of liquor is repeated as many times as may be required for the reduction of the interior pressure. By this gradual withdrawal of a considerable portion of the liquor originally charged upon thewood, a considerable saving of steam is efi'ected in view of the fact that the amount of liquor which has been gradually withdrawn from the digester, and consequently does not need to be heated to the higher temperature.

With a'view of roducing an unobjeetionable especially hig -grade pulp of particularly white colour it is of importance to ex-' pel the air as completely as possible from the digester before the cooking is finished. This requirement is accomplished in accordance with my' invention by the opening of the upper venting cock at a certain stage of the process: If, after this venting operation the interior pressure in the digester does not exceed thesum of the pressure of the sulphurous acid and steam at the respective temperature, this will be a reliable indication that no air isleft in the digester. Unless this result is accomplished with one venting operation, the venting should be repeated. There is no remarkable loss of sulphurous acid connected therewith, inasmuch as by the preceding withdrawal of considerable portions of the liquor from the'digester the liberation of free sulphurous acid has been reduced to a'minimuni.

After the venting operation the cooking process is carried through in the usual manner, and upon any exeessing of the admis' sible maximum pressure the required reduction of pressure is, always produced by the removal of a corresponding amount of liquor through the discharge 'valve at the bottom.

Example 1.

. pump is stopped. After that steam is admitted, and the pressure in the digesterwill increase to 4.2 atmospheres. By opening the bottom valve liquor is discharged until the excess pressure drops to 1.2 atmospheres. This drop of pressure from 4.2 to 1.2 atmos pheres is continued until a temperature of reduction of about 105 degrees C. is reached. Then the steam is shut in for say about 1% hours, with a simultaneous short venting by the top-valve, until the cooking pressure is equal to the sum of the pressure of sulphurous acid and the pressure of saturated steam corresponding to 105 degrees C., which partial pressures have been found by tests. Then the steam is again admitted, and the ressure from 4.2 to 3.2 atmospheres is produced by withdrawal of liquor, until the final tem erature of 138 degrees is reached. All further steps are carried out in the usual manner.

It should be pointed out that the process according to this invention is not restricted to the manufacture of pulp by the sulphite process, but it may also be employed, when the digestion is eiiected by alkalis or neutral salts. Nor is the invention restricted to the precise scheme of procedure and the elements described, but modifications may occur in accordance with local requirements and the convenience of the operator, and without deviating from the spirit of my invention, as pointed out in the claims, hereunto appended- I claim: 7

l. The process of manufacturing chemical wood pulp and the like from wood and similar material by digestion, which consists in keeping the digestion presslu'e at the admissiblc limit and reducing any excess of pres sure by withdrawing portions of the liquor, until the desired, temperature is reached, then removing the liberated air from the top of the digester, finishing the digestion, and preventing any further substantial rise of pressure.

2. The process of manufacturing chemical wood pulp'by digestion, which consists in reducing any encess of pressure by withdrawing portions of the liquor until the desired temperature is reached, then removing the liberated air from above the liquid, and reducing any excess of pressure by withdrawal of further quantities of liquor.

3. The process of manufacturing chemical wood pulp by digestion with a liquor, which consists in reducing excess of pressure beyond the admissible limit by withdrawing portions of the liquor until a temperature of about 105 degrees centigrade is obtained, then removing the liberated air from above the liquid, then finishing the digestion, and regulating the pressure by withdrawal of liquor during the finishing operation.

In testimony whereof I have signed this specification.

DR. ARMIN PEETZ. 

